Large public utility power plants produce electricity by coupling a generator to a mechanical power source such as a steam turbine. The generator comprises a rotor journaled in a stator, the rotor containing copper coil windings which carry a direct current for producing a magnetic flux. The power source spins the rotor at high speeds, causing the magnetic flux to sweep across copper coil windings in the stator and generate an electric current.
The generator stator comprises a cylindrical core having longitudinal slots along an inner surface, each slot containing a pair of top and bottom half turn coils. The top and bottom half turn coils emerge from the longitudinal slots at each end of the stator core and turn in order to connect with other half turn coils, thus forming a coil winding. The plurality of emerging end coils at each end of a stator core form an end winding basket. This basket arrangement of individual end coils must be consolidated into a unified structure in order to restrain radial and tangential coil movements which result from large electromagnetic forces, thermal expansion and coil vibration. Bracing assemblies are commonly used to secure end coils and reduce coil wear which results from coil movement during generator operation. In order to provide sufficient mass and stiffness for reducing steady state coil vibration and other sources of coil movement, the end coil bracing assemblies must be relatively large. As a result there are very limited clearances about the end winding basket and portions of the stator core are not readily accessible for maintenance and repair.
Recent improvements in bracing assemblies which further reduce coil movement in the end winding baskets of an electrical generator have used bracing systems in which a non-metallic segmented ring is positioned against upper surfaces of top end coils and a non-metalic cone shaped support ring is positioned against lower surfaces of bottom end coils to provide a continuous rigid support about the end winding basket. These rings are formed of a relatively rigid, baked fiber glass/epoxy resin composite. In order to minimize coil movement, such support rings may be substantially larger than other bracing systems which have been used to secure end coils against movement.
The cylindrical stator core, which may exceed 300 inches in length, is formed from a plurality of stacked punchings. A series of longitudinal through-bolts extending through the entire length of stacked punchings are used to compress the punchings between a pair of end plates and consolidate them into a unified structure. This arrangement minimizes vibration and stator core deformation which result from the large electromagnetic forces present during generator operation.
Generator designs typical of the prior art utilized the through-bolt clamping arrangement illustrated in FIGS. 1 and 2 for compressing the stacked stator core punchings. FIG. 1 illustrates, in partial cutaway, a generator 11 having a rotor 13 journaled within a stator core 15 within a support frame 17. As further illustrated in the partial cross-sectional view of FIG. 2, the stator punchings 19 are sandwiched between a pair of insulation washers 21,22 and a pair of steel compression plates 23,24 to distribute the compression forces. Through-bolts 25 having first and second threaded ends 27 and 29 are positioned to extend outward through the punchings 19 and the compression plates 23,24 on both the exciter end 33 and the turbine end 35 of the generator 11.
Procedures for stressing a stator core through-bolt 25 during generator assembly have involved first installing insulation washers 21,22, compression plates 23,34, insulation bushings 45,46, washers 43,44 and tensioning nuts 41,42 on the first and second bolt ends 27 and 29. The nuts are hand tightened against the stacked punchings 19 and the steel compression plates 23,24. Next, a tensioning nut 41,42 on one bolt end, e.g. the first end 27, is locked to the through-bolt 25 by deforming the bolt threads in order to prevent the nut from backing out. Then a hydraulic tensioning tool (not illustrated) is threaded onto the other bolt end, e.g. the second end 29, over the other tensioning nut 42. The through-bolt 25 is stressed with the tensioning tool to provide a desired compressive force against the stacked punchings 19. This creates a gap between the adjacent tensioning nut 42 and compression plate 24. The nut 42 is then threaded onto the bolt 25 to remove the gap and is locked in place by deforming the bolt threads. Finally, the tensioning tool is released so that the locked tension nuts 41,42 hold the through-bolt 25 in a stressed position to retain a compressive force against the stacked punchings 19. After all the through-bolts have been installed, half turn coils are placed in the longitudinal slots of the stator core and the end coils are interconnected to form a coil winding. With the winding in place, a bracing assembly is installed at each end of the turbine to secure the end coils into a unified structure.
This general procedure used for stressing through-bolts during stator core assembly is known to have several limitations affecting preventive maintenance of the stator core 15 and the degree of tensioning which can be obtained ,for the through-bolts 25. Notwithstanding the relatively large compressive forces used to form the stator core punchings 19 into a unified structure, the deformation and vibration forces incurred during generator operation have been known to cause individual punchings to slide against one another and to wear down their insulative coatings. Progressive wear may decrease the punching thickness and reduce the compressive forces provided by the stressed through-bolts 25. This in turn may result in greater levels of movement among the punchings and accelerate the overall wear process.
One solution for preventing these wear effects is to retension the through-bolts 25 if the compressive forces diminish to unacceptable levels. This is accomplished by again applying a compressive force with a tensioning tool. Next, one of the tensioning nuts 41,42 is again threaded inward to retain the new compressive force against the punchings after the tool is removed. However, this method of retensioning stator core through-bolts has been awkward and inconvenient due to limited accessibility and the large tensioning tools which must be positioned about the through bolt nuts. The task is especially difficult for generator designs in which the support rings are relatively large.
In the past, tensioning nuts have been locked in place by deforming the nut threads, as well as the bolt threads, during factory assembly in order to prevent backing out of the nuts 41,42. However, deformities in the nut threads and bolt threads adjacent the nuts must be machined in order to thread the nuts 41,42 inward for retensioning. Therefore, it has become a common practice not to deform the nut threads and to deform only the bolt threads up to one revolution in front of the nuts 41,42. This technique alleviates having to machine the nut and bolt threads when retensioning the through-bolts 25. A disadvantage of the technique is that it allows the nuts 41,42 to back out, up to one revolution of threading when they are installed. This effect results in a small, but significant, drop in the through-bolt compressive force at the time of installation. It is therefore desirable to provide a through-bolt assembly for compressing stator core punchings which does not involve the turning of tensioning nuts when retensioning the through-bolts.